This invention relates to a vinylidene fluoride resin film with a constant film thickness and excellent in both tearing strength and Young's modulus, a process for production thereof and a metallized film of the film.
A vinylidene fluoride resin film excellent in Young's modulus is disclosed in Japanese Laid-Open Patent Application No. 148628/1982 relating to an invention achieved by one of the present inventors. This film is obtained by melt extrusion of a vinylidene fluoride resin at a high draft ratio and cooling the extruded film on a cooling roller, and the distance between the die outlet and the cooling roller is thereby required to be made as short as 10 cm or less. The film obtained by this method is inevitably subject to shrinkage in width to some extent and therefore the physical properties differ between the both end portions and the central portion of the film in the plane which is vertical to the draft direction and parallel to the principal plane. For this reason, it has been necessary to provide only the portion uniform in physical properties by removing both ends by slitting. Besides, the film thus obtained has also the drawback of being readily torn along the drafted direction. More specifically, its tear strength is only 2 to 8 g.cm/cm as measured according to JIS-P8116 by means of a light load tear strength tester produced by Toyo Seiki K.K.
Further, Japanese Patent Application No. 173634/1982 (later laid open as Japanese Laid-Open Patent Application No. 62115/1984) discloses a film of a vinylidene fluoride polymer consisting mainly of vinylidene fluoride, having a crystalline structure of mainly an .alpha.-phase, having an orientation degree .pi. of the molecular axis in the crystalline region of 0.8-1.0 in a direction parallel with a film surface and being free of heat shrinkage due to heating from room temperature to the neighborhood of the melting point. The film developed by the research group of the present inventors is almost satisfactory in regard to a tear strength but only has a Young's modulus of 300 kg/mm.sup.2 at the most.